1. Technical Field
The present invention relates to a method and apparatus for integrating multipolar magnetic confinement with permanent magnet electron cyclotron resonance (ECR) plasma sources to produce highly uniform plasma processing for use in semiconductor fabrication and related fields.
2. Description of Related Art
Plasma producing devices are commonly employed in microelectronic device fabrication and similar industries requiring formation of extremely small geometrics. Plasma producing devices may be utilized in plasma-assisted processing to etch geometries into substrate or to deposit a layer or layers of material on the substrate.
One class of such plasma producing devices employs a magnetic field in conjunction with microwave energy. In these devices, plasma is produced from a working gas as a result of the interaction of a magnetic field with an electric field. A microwave waveguide may be employed to inject microwaves, which have an associated electric field, into an evacuable chamber containing the working gas. The microwaves propagate into the chamber in a direction substantially perpendicular to the surface of the workpiece. The electric field associated with the microwaves is perpendicular to the direction of the propagation, radially outward from a line 1. following the direction of the propagation of the microwaves. Plasma ions from the working gas are accelerated by the electric field along such radial lines.
A magnetic field is provided in a direction generally aligned with the direction of microwave propagation, causing plasma electrons within the working gas to rotate around the direction of microwave propagation at right angles with the magnetic field. At the plane of resonance, the point at which the electric field associated with the microwave energy and the rotation of plasma electrons are in phase, the microwave electric field constantly accelerates the rotating plasmas electrons. The energy of this acceleration dissociates molecules of the working gas into atoms and removes electrons from the atoms, creating ions and additional electrons. The ions then diffuse and impinge upon the exposed surface of the workpiece.
The requisite magnetic field may be provided by a single permanent magnet situated above the outlet of the microwave waveguide into the chamber. An adjusting element may be provided to vary the spatial relationship between the magnet and the waveguide opening, thus altering the location of the plane of resonance or xe2x80x9cresonance zonexe2x80x9d within the chamber.
While use of a permanent magnet situated over the waveguide opening to the chamber has advantages over other plasma producing methods, a permanent magnet of the size required to provide the requisite magnetic field may be very expensive. Additionally, plasma uniformity across the surface of the workpiece is generally necessary to achieve etched goemetrics or deposited layers having relatively uniform dimensions from the center to the periphery of the workpiece surface. Prior art attempts to obtain plasma uniformity have focused on achieving a uniform magnetic field, which requires very large and bulky magnets. Another drawback of the use of permanent magnets in plasma producing devices relates to the necessity of positioning the microwave waveguide between the permanent magnet and the workpiece. This constrains placement of the permanent magnet with respect to the chamber, and as the magnet face is moved further from the chamber, larger, more expensive magnets are required to produce the requisite magnetic field.
One solution to these difficulties is described in U.S. patent application Ser. No. 08/770,316, which teaches the use of a reduced height waiveguide with a rectangular-to-circular transmission mode converter, allowing the face of the permanent magnet to be moved closer to the resonance zone. Since field strength is inversely related to the square of the distance (xcx9c1/d2) for a dipole, this placement of the magnet closer to the resonance zone allows a smaller, less powerful magnet to realize the same magnetic field in the resonance zone. Although permanent magnets and electromagnets do not provide magnetic fields which are precisely those of a magnetic dipole, both provide fields having predominantly dipole characteristics. Closer to a permanent magnet, more substantial deviations from a dipole may be found.
The above-identified patent application also discloses the benefits of employing a return piece (also called a xe2x80x9cpole piecexe2x80x9d) adjacent to the pole of the permanent magnet which is opposite the waveguide and evacuable chamber. The return piece, which may be composed of soft iron, short circuits the magnetic field emanating from the surface of the permanent magnet adjacent to the return piece, increasing the xe2x80x9ceffective heightxe2x80x9d or perceived magnetic strength of the permanent magnet. The return piece compensates for magnetic flux which would be generated by a larger magnet, allowing a smaller permanent magnet to produce a magnetic field having the strength equal to that produced by a similar magnet (without the return piece) of a larger height. Since the magnetic materials are typically very expensive, the ability to utilize a smaller permanent magnet to produce the requisite magnetic field at the resonance zone improves the commercial viability of the plasma producing device. Employing a return piece also reduces stray magnetic fields beyond the return piece.
The return piece described, particularly with optional sidewalls extending from the return piece and forming a xe2x80x9csheathxe2x80x9d around the permanent magnet, provides some ability to shape and direct the magnetic field produced by the permanent magnet, and to minimize stray magnetic fields around the magnet. Control over both the direction of the magnetic field lines produced by the permanent magnet in the resonance zone and stray magnetic fields may thus be achieved. Additionally, the magnetic field near the workpiece may be minimized, resulting in a inherent increase in plasma uniformity.
Permanent magnet ECR systems and methods improve upon plasma-processing methods and hardware. The ECR systems and methods have been demonstrated to facilitate decreasing the amount of permanent magnet material required. Further, they have successfully demonstrated the creation of more uniform magnetic fields in the resonance zone and the ability to reduce stray fields in the process chamber. Additional information regarding the PlasmaQuest ECR Method can be found in U.S. patent application Ser. No. 09/019,573 titled xe2x80x9cPermanent Magnet ECR Plasma Source with Magnetic Field Optimizationxe2x80x9d filed on Feb. 6, 1998, which is incorporated herein for all purposes.
Multipolar magnetic confinement involves the use of multiple magnets arranged about the periphery of the evacuable chamber. Multipolar magnetic confinement has been used by PlasmaQuest as a means of reducing wall losses in low pressure plasmas and has been implemented on a variety of low pressure plasma systems. In plasma systems which do not require magnetic field for operation, magnetic confinement typically consists of an array of small permanent magnets with opposing polarities arranged around the periphery of the chamber. In systems which require magnetic fields for operation, such as ECRs and helicon wave plasma systems, the confinement magnets are in addition to the magnets required for ECR or helicon wave excitation. The confinement magnet fields are typically small relative to the ECR or helicon magnetic field.
Charged particles are confined to travel along the magnetic field lines by the xcexdxc3x97B force; thus magnetic fields have a significant impact on plasma diffusion. This impact is the fundamental basis of magnetic confinement. When used in the semiconductor fabrication industry, plasma reactors require not only high plasma density in the generation region, but at the wafer surface as well.
The optimal integration of permanent magnet ECR sources with an integrated multipolar magnetic confinement has yet to be achieved in the prior art. Therefore, a need exists to combine, in an optimum arrangement and condition for a given application, multipolar magnetic confinement along with permanent magnet ECR plasma sources. This combination should optimize plasma extraction from the source, optimize plasma uniformity, maximize the area of uniform plasma density, and allow for tuning between a centrally peaked and centrally hollow plasma profile.
The present invention provides a method and apparatus for integrating multipolar magnetic confinement with permanent magnetic electron cyclotron resonance (ECR) plasma sources to produce highly uniform plasma processing is disclosed. This invention is used in semiconductor fabrication and related fields when uniform plasma processing is desirable.
In a preferred embodiment, the plasma processing apparatus includes a vacuum chamber, a workpiece stage within the chamber, a permanent magnet electron cyclotron resonance plasma source directed at said chamber, and a system of permanent magnets for plasma confinement about the periphery of said chamber. The permanent magnets for plasma confinement are arranged in a multiplicity of rings with the plane of the rings perpendicular to the vacuum chamber axis and to the direction of propagation of the microwave into the vacuum chamber. The number of rings is chosen with respect to the vacuum chamber dimensions to produce a large, low magnetic field region in the region of the vacuum chamber adjacent to the workpiece stage.
Multiple magnet configurations are disclosed to provide an optimal confinement magnet design in combination with field effect of the ERC magnet. This invention provides for superior uniformity in plasma processing over the prior art. The invention also optimizes plasma extraction from the source, maximizes the area of uniform plasma density, and provides for tuning between centrally peaked and centrally hollow plasma profiles.
The above as well as additional features and advantages of the present invention will become apparent in the following written detailed description.